Regular or Premium

The general Seca II forum. XJ600 centric but all motorbikes welcome!
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Oddball
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Motorcycles Owned Currently: 1996 Yamaha XJ600 (Seca II)

Fri Aug 04, 2017 10:49 pm

I have read somewhere - perhaps this forum, although I can't refind it - that someone suggested his XJ600 runs "much better" on premium gas. He even said it got slightly better mileage. Can anyone second those claims? I've always run regular in mine and it seems to work dandy. But of course we all want a little more oomph/smoothness/mileage - especially if it's as easy as paying a few more cents for a tankful of premium.

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Crimson
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Sat Aug 05, 2017 6:40 am

I think it's doubtful. But one theory is that a cylinder head and/or piston is caked up with carbon deposits, it may well have hotspots, and the irregular surface area might cause ignition before the cylinder head has had time to completely fill up with air-and-fuel mixture. As higher octane fuel is slightly slower to ignite than lower octane fuel, it has slightly more time before the mixture ignites. In a caked-up cylinder head, this may make a lot of difference, as you would get in more mixture before the burn starts. All kinds of factors influence burn rate and burn efficiency, in the cylinder head. Even the position of the spark plug matters. So this effect may be more noticeable with dirtier engines.

I run regular, except for every 5 or so fill-ups, when I run premium. This is only because the higher octane fuel where I live has cleaners added to it.
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radare
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Sat Aug 05, 2017 9:44 am

I use premium (91 octane here in Denver) to prevent detonation when the engine is really hot. I don't think it's required.

That's really all high octane fuel does is prevent pre ignition. The higher or hotter the combustion charge, the higher octane is required.

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mikee112
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Sat Aug 05, 2017 12:32 pm

I started using super unleaded in the UK a couple of years ago, I read somewhere that super unleaded contains LESS Ethanol than premium ( the rubber parts in the carbs and intake system don't like Ethanol :( ).... whether or not it makes a difference to performance is debatable, I think the bike runs a bit smoother but that maybe just my imagination compensating for the extra cost :???:

EDIT: Premium unleaded petrol here in the UK is 95 octane and super unleaded is 97/98 octane, ( we only have premium and super, we don't have regular ! ) so it seems our lowest grade contains more octane than the American premium grade... or maybe it's a different way of measuring ?
the Euro measurement is RON

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Siper2
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Motorcycles Owned Currently: 1994 Yamaha XJ600SF

Sat Aug 05, 2017 5:06 pm

I've always used regular, which is 87 where I am. radare makes a good point about hot temps... maybe I'll bump it up to 89. *shrug*

Pretty sure the factory manual suggests regular.

S2

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radare
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Sat Aug 05, 2017 8:26 pm

mikee112 wrote:
Sat Aug 05, 2017 12:32 pm
EDIT: Premium unleaded petrol here in the UK is 95 octane and super unleaded is 97/98 octane, ( we only have premium and super, we don't have regular ! ) so it seems our lowest grade contains more octane than the American premium grade... or maybe it's a different way of measuring ?
the Euro measurement is RON
We use the average of RON and MON in the US (RON+MON)/2. Many of the lower altitude states sell 89/91/93. As elevation increases, the octane levels sold also decreases. In Colorado, we get 85/87/91. We can do this because the elevation is higher and less air reduces the chances of detonation and so the lower octane is permissible. Another note; here in Colorado, we use Ethanol blended gasoline which contains 10% ethanol. It's done to reduce certain smog emissions.

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Unit562
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Fri Aug 11, 2017 7:41 pm

As an automotive tech, I can tell you that at the CR these bike run, you're going to cause your self a lot of problems with lower octane fuel. Lower octane also means lower quality of fuel. I rebuild hundreds of engines a year, use, high, octane, fuel. Also use quality fuel. I only use Fred Meyer out here (local grocery store chain owned by Kroger) because they source their fuel from a local fueling company that has a contract with Chevron.
One bad idea fuels most Bike Nights.
My Civic isn't a lawnmower, it's a weed whacker. Get it right, gosh.

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TwoWheelGeorge
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Fri Sep 22, 2017 11:13 am

Radar, you made probably the best point... ethanol. Ethanol is corn and will gum up your carbs. On newer bikes it's a non issue, but on a carb bike it matters. Also, the xj6 is not that high in compression where it will actually benifit from higher octane. You will not get detonation. So the lower octane I feel is benifical given those criterion. Not to mention lower fuel costs.

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radare
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Mon Sep 25, 2017 12:17 am

TwoWheelGeorge wrote:
Fri Sep 22, 2017 11:13 am
Radar, you made probably the best point... ethanol. Ethanol is corn and will gum up your carbs. On newer bikes it's a non issue, but on a carb bike it matters. Also, the xj6 is not that high in compression where it will actually benifit from higher octane. You will not get detonation. So the lower octane I feel is benifical given those criterion. Not to mention lower fuel costs.
We can't get non-ethanol gas here. So I've run thousands of gallons of it through motorcycles and not yet had a gummed-up-carb issue that wasn't the result of a bike sitting for 2 or more years with fuel in its bowls. Don't really think octane matters much with these bikes unless you are specifically getting detonation. I run 91 in mine, though . . .

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